On 8/22/16 7:06 PM, Darrick J. Wong wrote: > [add Dave and Christoph to cc] > > On Mon, Aug 22, 2016 at 04:14:19PM -0400, Jeff Mahoney wrote: >> On 8/21/16 2:59 PM, Tomokhov Alexander wrote: >>> Btrfs wiki FAQ gives a link to example Python script: >>> https://github.com/stsquad/scripts/blob/master/uncow.py >>> >>> But such a crucial and fundamental tool must exist in stock btrfs-progs. >>> Filesystem with CoW technology at it's core must provide user sufficient >>> control over CoW aspects. Running 3rd-party or manually written scripts for >>> filesystem properties/metadata manipulation is not convenient, not safe and >>> definitely not the way it must be done. >>> >>> Also is it possible (at least in theory) to "uncow" files being currently >>> opened in-place? Without the trickery with creation & renaming of files or >>> directories. So that running "chattr +C" on a file would be sufficient. If >>> possible, is it going to be implemented? >> >> XFS is looking to do this via fallocate using a flag that all file >> systems can choose to honor. Once that lands, it would make sense for >> btrfs to use it as well. The idea is that when you pass the flag in, we >> examine the range and CoW anything that has a refcount != 1. > > There /was/ a flag to do that -- FALLOC_FL_UNSHARE_RANGE. However, > Christoph and Dave felt[1] that the fallocate call didn't need to have > an explicit 'unshare' mode because unsharing shared blocks is > necessary to guarantee that a subsequent write will not ENOSPC. I > felt that was sufficient justification to withdraw the unshare mode > flag. If you fallocate the entire length of a shared file on XFS, it > will turn off CoW for that file until you reflink/dedupe it again.
Is that a flag or just that it's reverting to "normal" XFS operation? We have a nocow flag for btrfs, but it's more like nocow* because it's still possible to create new references to the extents and those must be CoW'd later. I think that's about all we can offer since the nocow flag set otherwise would imply that /every/ snapshot does a full copy of anything marked nocow and I don't think that's the expectation either. > At the time I wondered whether or not the btrfs developers (the list > was cc'd) would pipe up in support of the unshare flag, but nobody > did. Consequently it remains nonexistent. Christoph commented a few > months ago about unsharing fallocate over NFS atop XFS blocking for a > long time, though nobody asked for 'unshare' to be reinstated as a > separate fallocate mode, much less a 'don't unshare' flag for regular > fallocate mode. > > (FWIW I'm ok with not having to fight for more VFS changes. :)) Agreed. :) >> That code hasn't landed yet though. The last time I saw it posted was >> June. I don't speak with knowledge of the integration plan, but it >> might just be queued up for the next merge window now that the reverse >> mapping patches have landed in 4.8. > > I am going to try to land XFS reflink in 4.9; I hope to have an eighth > patchset out for review at the end of the week. > > So... if the btrfs folks really want an unshare flag I can trivially > re-add it to the VFS headers and re-enable it in the XFS > implementation <cough> but y'all better speak up now and hammer out an > acceptable definition. I don't think XFS needs a new flag. Thanks for the explanation, Darrick. I'm not advocating for a flag. That was just the last state of the implementation as I remember it. I missed the discussion turning to not needing it at all. I suppose the only thing missing, and this applies to both XFS and a future btrfs implementation, is documentation for the user explaining that the guarantees fallocate makes about ENOSPC not failing are voided by operations that can re-share extents. -Jeff -- Jeff Mahoney SUSE Labs
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